Abstract
BackgroundFire plays an important role in controlling the cycling and composition of organic matter and nutrients in terrestrial and aquatic ecosystems. In this study, we investigated the effects of wildfire severity, time since fire, and site-level characteristics on (1) concentration of multiple solutes (dissolved organic carbon, DOC; total dissolved nitrogen, TDN; dissolved organic nitrogen, DON; calcium, Ca2+; magnesium, Mg2+; potassium, K+; sodium, Na+; chloride, Cl−; nitrate, NO3−; ammonium, NH4+; sulfate, SO42−; and phosphate, PO43−), and (2) the molecular composition of stream-dissolved organic matter (DOM) across 12 streams sampled under baseflow conditions in Yosemite National Park, California, USA. Samples were collected from low- and high-severity burned stream reaches, as well as an unburned reference stream reach.ResultsFire severity, time since fire, and variability in site-level characteristics emerged as the strongest influences on streamwater chemistry. Results from mixed-effect models indicated that DOC and DON concentrations decreased with time since fire in high-severity burned stream reaches. In low-severity burned stream reaches, DOC concentrations increased, and DON concentrations slightly decreased with time since fire. We also found that declines in aromaticity (expressed as decreased SUVA254) and mean molecular weight DOM (expressed as increased E2:E3 ratios) with time since fire were associated with high-severity fires. Mixed-effect models also indicated that site-level characteristics played a role in solute responses. Aliphatic structures dominated streamwater DOM composition across fire-impacted catchments, but neither fire severity nor time since fire was a significant predictor of the proportion of aliphatic structures in streamwater DOM. North aspect exhibited the highest concentrations of Ca2+, K+, and Mg2+, whereas the north-northwest aspect exhibited the highest concentrations of Cl− and SO42+. We also observed elevated Ca2+, K+, and Mg2+ in burned (but not reference) stream reaches with pool-riffle versus step-pool bed morphology.ConclusionsTaken together, our findings suggest that the response of stream chemistry to wildfires in the Sierra Nevada, California, can persist for years, varying with both fire severity and site-specific characteristics. These impacts may have important implications for biogeochemical cycles and productivity in aquatic ecosystems in fire-adapted landscapes.
Highlights
Wildfire plays a pivotal role in controlling the dynamics of organic matter (OM) and inorganic solutes in both terrestrial and aquatic ecosystems
Differences in streamwater solute concentrations between burned streams and the reference stream Visual assessment of stream solute concentrations in the reference stream (Table 2, Fig. 3) showed that streams flowing through riparian zones that burned with high severity exhibited elevated Ca2+, Na+, K+, and PO43− concentrations (Fig. 3a, b, d, and l)
For high-severity fire, dissolved organic carbon (DOC) and Dissolved organic nitrogen (DON) concentrations decreased with time since fire, whereas in low-severity fire, DOC concentrations increased and DON concentrations slightly decreased with time since fire
Summary
Wildfire plays a pivotal role in controlling the dynamics of organic matter (OM) and inorganic solutes in both terrestrial and aquatic ecosystems. Fire can alter hydrologic flow paths in soil and increases susceptibility of soils to erosion (Spigel and Robichaud 2007, Florsheim et al 2017) These changes increase the rate at which sediments, dissolved and particulate carbon (C), and nutrients are transported from post-fire landscapes and delivered to fluvial networks (Moody et al 2013, Dahm et al 2015, Mast et al 2016, Abney et al 2017, Esposito et al 2017). Samples were collected from low- and high-severity burned stream reaches, as well as an unburned reference stream reach
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